An integral part of human sensory capability is the ability to localize sound sources. For example, the ability to localize sound sources allows for improved communication and increased safety. Communication may be improved, for example, by distinguishing sound sources by their location and by using location information to turn toward a person who is talking. Safety may be enhanced, for example, by localization in that an approaching hazard, such as an approaching car or animal, may be located before it becomes an imminent danger.
The human auditory system's ability to localize sound is believed to be based on several aspects of how sound reaches the left and right ears of the listener. Among these aspects are interaural time differences (ITD), interaural phase differences (IPD), and interaural level differences (ILD). The way in which a listener's head, shoulders, and ears filter sounds in a location and frequency dependent manner is referred to as the head-related transfer function (HRTF). The ability of a listener to localize sound by interpreting the HRTF is believed to be a learned response.
There are several situations where humans may suffer from reduced localization capability or completely lose their ability to localize sound sources. One such situation is complete loss of hearing in both ears. In such individuals, no sounds can be heard, and thus no unaided localization can occur.
Another situation is Single-Sided Deafness (SSD), where an individual suffers from complete hearing loss in one ear while retaining some level of hearing in the other ear. These individuals generally are unable to locate the source direction of a sound without some auxiliary information. Such auxiliary information may include visual cues and/or listening during physical movement. For example, a person with SSD may move around and make sound source estimations based on the strength of the sound being heard while in various locations or while facing various directions. However, such localization requires the sound source to occur over a substantial amount of time to enable the listener to move around. Individuals suffering from unilateral hearing loss (where one ear has some level of hearing loss, while the other ear is normal), may suffer from similar localization deficiencies as individuals with SSD.
Individuals suffering from bilateral hearing loss (where both ears have some level of hearing loss), may also have difficulty in localizing sound sources as compared to people with normal hearing.
Moreover, hearing aid users may have poorer ability to localize sound sources when wearing their hearing aids than without their hearing aids. Hearing aids typically reproduce sound such that the wearer perceives sound sources to be localized inside the head, and as such localization capabilities may be reduced or eliminated.
When the ability to localize sound sources is reduced or lost, individuals may experience greater cognitive loading during conversation, particularly when the conversation is among more than two people. Furthermore, such individuals may be slower to react to environmental dangers as compared to individuals with normal localization capabilities. Moreover, individuals with reduced localization capabilities may find some social activities, such as sports or group activities, more difficult.